As is commonly known, the available AC input voltage varies from area to area. Presently, the usual AC electrical source used in Korea and Japan is 100 V and/or 220 V. In some countries, including Australia, 250 V of AC power is used, and AC power from 100 V-250 V is usually used worldwide. (Table I below lists by country the type of available AC power.)
Therefore, when electronic appliances are exported to other countries where the AC electrical source has a different voltage, a power supply using a transformer having a fixed voltage specification, depending on the area where it is going to be exported, is usually necessary.
This variation in AC power from country to country causes inconvenience in making electronic appliances and thereby results in both increase of production costs and other additional problems.
A conventional power supply is illustrated in FIG. 1, wherein a tap-switch SW2 is switched to one of two positions as determined by the input voltage of the available alternating current (for instance, 100 V, 220 V) which is to be inputted into input terminals 1a and 1b. At the output side of the step-down transformer T, a predetermined AC output voltage is obtained when the switch SW1 is closed. This output voltage is rectified by a resistor Ra and a rectifier bridge Rf and then is partially smoothed by a capacitor Ca. This partially smoothed DC voltage is further filtered by an active filter circuit comprising transistors Qa, Qb and Qc, for the purpose of removing ripple caused by the frequency of AC voltage.
However, in the conventional circuit mentioned above, a step-down transformer and a tap-switch (for setting of either 100 V or 220 V) is necessary. The conventional circuit is usually included inside an electrical appliance (for example, TV, audio set, video set, etc.) in case of a variation of AC input voltage depending on the area where the appliance is used. Frequently, in order to set the tap-switch, the appliance cabinet must be disassembled. If it is not properly set, troubles in electronic appliances may occur. Also, this design may cause difficulties in handling, as well as increasing the cost of production and the weight of the appliance. Further, because some electrical power is lost in the transformer T, its efficiency is very low. Efficiency of DC output compared to AC input is about 70 percent and the remaining 30 percent appears as a loss of electrical power in the circuit.